WordPress.org

1. Upload the FOLDER 'wp-page-numbers' to the /wp-content/plugins/
2. Activate the plugin 'WP Page Numbers' through the 'Plugins' menu in admin
3. Go to 'Options' or 'Settings' and then 'WP Page Numbers' to change the options

Usage

Replace the the 'nextpostslink()' and 'previouspostslink()' with the code below in your theme (archive.php, index.php or search.php).

<?php if(function_exists('wp_page_numbers')) { wp_page_numbers(); } ?>